Ferromagnetic materials can have magnetostrictive properties that can cause the materials to change shape in the presence of an applied magnetic field. The inverse can also be true. When a stress is applied to a conductive material, magnetic properties of the material, such as magnetic permeability, can change. A magnetostrictive sensor can sense the changes in magnetic permeability and, because the changes can be proportional to the amount of stresses applied to the conductive material, the resulting measurement can be used to calculate the amount of stress.
The changes in the magnetic permeability arising from an application of stress to the conductive material, however, can be small, making accurate measurement difficult. Some magnetostrictive sensors can be manually aligned and a gap is set by a gauge. Such alignment can result in different air gaps being defined between each detector pole of the sensor.